Re: [XEN PATCH] tools: add x2APIC entries in MADT based on APIC ID

[Jan Beulich <jbeulich@suse.com>]

On 26.03.2024 16:57, Matthew Barnes wrote:
>>> This patch scans each APIC ID before constructing the MADT, and uses the
>>> x2APIC entry for each vCPU whose APIC ID exceeds the size limit imposed
>>> by regular APIC entries.
>>
>> It is my understanding that if you use any x2APIC entry, every CPU needs
>> to have one.
> 
> In the ACPI 6.4 specification, section 5.2.12.12, the note says the following:
> 
> [Compatibility note] On some legacy OSes, Logical processors with APIC ID
> values less than 255 (whether in XAPIC or X2APIC mode) must use the Processor
> Local APIC structure to convey their APIC information to OSPM, and those
> processors must be declared in the DSDT using the Processor() keyword.
> 
> Therefore, even in X2APIC mode, it's better to represent processors with APIC
> ID values less than 255 with APIC entries for legacy reasons.

Well, my reading of that is different: All CPUs need to have x2APIC entries
if one has, and CPUs with small enough APIC IDs _additionally_ need xAPIC
entries. That's what I've been observing on real hardware, too.

Jan